Novel cationic aryl bithiophene/terthiophene derivatives as corrosion inhibitors by chemical, electrochemical and surface investigations

Two novel bithienyl fluorobenzamidine derivatives namely, 4-([2,2′:5′,2′′-terthiophen]-5-yl)-2-fluorobenzamidine hydrochloride salt (MA-1615), 5′-(4-amidino-3-fluorophenyl)-[2,2′-bithiophene]-5-carboxamidine dihydrochloride salt (MA-1740) were synthesized, characterized and their corrosion inhibition properties were evaluated by electrochemical methods for carbon steel (C-steel) in 1 M HCl. Experimental investigations revealed that the inhibition effectiveness of the investigated inhibitors (INHs) by the Tafel polarization method followed the order: MA-1740 (96.9%) > MA-1615 (95.6%), demonstrating higher efficiency than inhibitors of similar structure reported in the literature. The investigated bithiophene derivatives exhibit mixed-type corrosion inhibition characteristics by blocking the active sites on the surface of C-steel. EIS study revealed that the INHs behave as interface-type corrosion inhibitors. UV–Visible spectrometric measurements confirmed a complex formation between the Fe2+ cation released during the corrosion reactions and inhibitor molecules.


Methodology for preparation of inhibitors. Monocationic terthiophene derivative 4. 4-(5′-Bromo-
[2,2′-bithiophen]-5-yl)-2-fluorobenzonitrile (2). Bromobithiophene derivative 2 was prepared starting from 4-([2,2′-bithiophen]-5-yl)-2-fluorobenzonitrile (1) 21 , on treatment with NBS in DMF adopting the reported methodology 21 . Bromobithiophene compound 2 was obtained as a yellow solid in 87% yield, m.p. 170-171 °C (EtOH). R f = 0.72, petroleum ether-EtOAc (8:2). IR (KBr) ν/cm −1 ; 3088 (CH, stretch), 2230 (CN, stretch), 1615, 1555, 1520 (C=C, stretch). 1    was freshly prepared by dilution of analytical grade HCl with a concentration of 37% using bi-distilled water. The concentration of MA-1615 and MA-1740 inhibitors in corrosive solution ranged from 1 (1 × 10 6 M) to 500 µM (5 × 10 4 M). Volta lab 80 (Tacussel-Radiometer PGZ-402) was used for electrochemical tests {Potentiodynamic polarization measurements (PP) and Electrochemical impedance spectroscopy (EIS)}. Electrochemical tests were executed by the typical 250 ml sealed electrochemical glass cell containing a finely cut API 5L X70 grade C-steel as working specimen, with an exposed surface area for 1 cm 222 . Platinum metallic sheet was mounted as the auxiliary electrode, and saturated calomel electrode (SCE) was utilized as the reference elec- www.nature.com/scientificreports/ trode. The surface of the working electrode was polished to mirror finish with various grades of emery papers (from 320 to 2000), degreased by acetone, cleaned by demineralized water and finally dried at room temperature before starting the tests. The reference electrode was placed close to the working electrode surface using Luggin-Haper capillary to nullify the iR potential drop between both the electrodes ends. EIS measurements were conducted via AC signal (10 mV) peak to peak, in potentiostatic mode at open-circuit steady state potentials, in the frequency extent between 100 kHz and 20 mHz 23 . Potentiodynamic polarization (PP) tests were executed using the scan rate at 1 mV s −1 and sweeping the electrode potential ± 300 mV relative to the steady state potential 24 . To ensure the consistency and accuracy of the results, all tests had been repeated three times.
Weight loss measurements. Tests were performed on petroleum pipeline alloy API 5L X70 carbon steel with density, D = 7.86 g cm −3 and with the same chemical composition of the electrode material. The carbon steel coupons were fabricated with dimension 3 × 2 × 0.2 cm. The surface of tested coupons was abraded using various grades of emery papers (from 320 to 2000), degreased with acetone, cleaned by demineralized water and finally dried at room temperature before starting the weight loss experiments. After weighing accurately, the specimens were immersed in 250 ml beaker, which contained 150 ml aggressive solution with and without addition of different concentrations of the tested inhibitors. After the required immersion time (48 h), the test specimens were removed, washed with double distilled water, dried by a jet of air and finally weighed. The surface coverage area (θ) for the different concentrations of the investigated inhibitors in 1 M HCl "aggressive solution" and the inhibition efficiency from weight loss, η w (%) were calculated by the following Eq. (1): where, ΔW and ΔW i are the weight loss per unit area in absence and presence of additive, respectively. The rate of corrosion (K) in (mg cm −2 h −1 ) was calculated by the following Eq. (2): where, ΔW is the average weight loss of three parallel carbon steel sheets in mg, S is the total area of the specimen in cm 2 and t is the immersion time in hour.
UV-visible spectra measurement. UV-visible absorption spectrometric method was carried out for (500 µM) of inhibitor solution, and the corrosive solution containing (500 µM) of investigated inhibitor after immersion of C-steel sample at 30 °C for 28 h. The spectra were measured using a PG instruments T80+ spectrometer.

Morphology analysis.
Morphological investigation was carried out by Nanosurf Flex AFM for API X70 type C-steel samples, with dimensions 10 × 10 × 2 mm. The API 5L X70 steel surface was pretreated in the identical way of the working electrode and immersed for 24 h in 1 M HCl solution without and with 0.001 M of MA-1740 at 298 ± 1 K. Samples were collected, and then dried for analysis with AFM 25 .

Results and discussion
Synthesis and structure elucidation of inhibitors. Monocationic terthiophene derivative 4. The preparation of the new terthienylbenzamidine derivative 4 ( Fig. 1) begins with bromination reaction of bithiophene derivative 1 21 with NBS to afford bromo bithiophene compounds 2 followed by a Stille coupling reaction with 2-tributyltin thiophene to furnish the terthienylbenzonitrile 3. Benzonitrile derivative 3 was allowed to react with lithium bis-trimethylsilylamide, followed by hydrolysis with hydrogen chloride. The resultant precipitate was neutralized with 1 N NaOH to furnish the corresponding free base of terthienylbenzamidine 4. The terthienylbenzamidine hydrochloride salt 4 was prepared by treatment of the proper free base of monoamidine with hydrogen chloride in ethanol. Structure of the newly synthesized terthienylbenzonitrile derivative 3 was assigned based on its elemental and spectral analyses. IR spectra of the terthienylbenzonitrile 3 indicated the presence of nitrile group with stretching vibrations at 2228 cm −1 . 1 H-NMR spectrum of the fluorobenzonitrile derivative 3 displayed seven signal integrated for seven protons of terthienyl moiety and two signals integrated for three protons of 1,3,4-trisubstituted benzene ring. Mass spectrum of compound 3 gave a molecular ion peak m/z at 367 (M + ) as a base peak. Structure of the newly synthesized terthienybenzamidine derivative 4 was assigned based on its elemental and spectral analyses. IR spectra of compound 4 indicated the disappearance of nitrile group and displayed new peaks corresponding for N-H stretching vibrations at ν′ 3460, 3389 cm −1 . 1 H-NMR spectrum of the monocationic compound 4 gave two singlet signals at δ 9.38 (2H, NH 2 ) and 9.49 (2H, + NH 2 ) of the cationic amidine group, plus five signals integrated for ten protons of terthienyl moiety and 1,3,4-trisubstituted benzene ring. 19 F-NMR displayed a singlet signal at − δ112.88 ppm (using TFA as external standard) of fluorine atom. Mass spectrum of terthienyl fluorobenzamidine derivative 4 gave a m/z peak at 384 of its molecular ion peaks (M + ), and m/z peak at 385 (M + + 1) as a base peak, along with a fragment with m/z peak at 367 due to loss of a molecule of ammonia.
Dicationic bithiophene-5-carboxamidine derivative 6. The preparation of the new bithiophene diamidine 6 ( Fig. 2) begins with cyanation of 4-(5′-bromo-[2,2′-bithiophen]-5-yl)-2-fluorobenzonitrile (2) with Cu(1)CN in DMF at 120-130 °C to afford the dicarbonitrile derivative 5. Dicarbonitrile derivative 5 was allowed to react with LiN(TMS) 2 , followed by hydrolysis with hydrogen chloride. The resultant precipitate was neutralized with 1 N www.nature.com/scientificreports/ NaOH to furnish the corresponding free base of diamidine 6. The diamidine hydrochloride salt 6 was prepared by treatment of its free base of diamidine with hydrogen chloride in ethanol. Structure of the newly synthesized bithiophene dicarbonitrile derivative 5 was assigned based on its elemental and spectral analyses. IR spectra of the bithiophene dinitrile 5 indicated the presence of nitrile group with stretching vibrations at 2212 cm −1 . 1 H-NMR spectrum of compound 5 displayed four doublet signals (one proton each) of bithiophene moiety at δ 7.57, 7.67, 7.89, 7.98 ppm, plus two signals integrated for three protons of 1,3,4-trisubstituted benzene moiety. Mass spectrum of compound 5 gave a molecular ion peak m/z at 310 (M + ) as a base peak. The newly synthesized bithienylbenzamidine derivative 6 was assigned based on its elemental and spectral analyses. IR spectra of the diamidine 6 indicated the disappearance of the carbonitrile group and displayed new peaks corresponding for N-H stretching vibrations at ν′ 3342, 3187 cm −1 . 1 H-NMR spectrum of compound 6 gave four singlet signals at δ 9.22 (2H), 9.45 (2H), 9.51 (2H), 9.54 (2H) characteristic for the cationic diamidine groups, four doublet signals at δ 7.65 (1H), 7.66 (1H), 7.89 (1H), 8.13 (1H) of bithiophene-H′s, plus two signals integrated for three protons of 1,3,4-trisubstituted benzene moiety. 19 F-NMR displayed a singlet signal at − δ112.76 ppm (using TFA as external standard) of fluorine atom. Mass spectrum of bithiophene  Table 3. It is obvious that with increasing the inhibitor concentration, the generated corrosion current density (i corr ) for the API 5L X70 grade C-steel sample in the acidic medium shows a decreasing trend. This behaviour demonstrates the ability of the cationic aryl bithiophene/terthiophene derivatives to increase in the inhibition efficiencies and to reduce the corrosion rate caused via acidic solution. Looking at the PP plots showed that the anodic and cathodic lines, it is obvious that the curves tend to decline, which is an indication of the adsorption of cationic aryl bithiophene/terthiophene derivatives on the C-steel surface lower cathodic hydrogen evolution and anodic dissolution of Fe ions. Additionally, the difference in the values of Tafel slopes (β a and β c ) with different doses of cationic aryl bithiophene/terthiophene derivatives was small, indicating that the cationic aryl bithiophene/ terthiophene derivatives hindered the corrosion of API 5L X70 type C-steel via inhibiting both anodic and cathodic reactions, without changing the corrosion process mechanism 27,28 . Tafel polarization was carried out at different temperature for the dicationic bithiophene inhibitor MA-1740 at optimum concentration of study as in Fig. 6. The corrosion measurements by Tafel polarization at different temperature are summarized in Table 4. From the results, it is obvious that the current density is higher when increasing the temperature, and so does the desorption rate of compounds on metal surface. However, this has no big shift in plots as can be seen in Fig. 6, this indicates no change in adsorption mechanism of compounds. The adsorption of cationic aryl bithiophene/ terthiophene derivatives on API 5L X70 can be attributed to the interaction between the lone pairs of sulfur and nitrogen atoms present in these derivatives and the empty d-orbital of API 5L X70-type C-steel surface via coordination bonds. The resulting protection was achieved with the creation of a barrier film and led to inhibit corrosion. The inhibition efficiency (η p ) can be calculated by the following equation 29,30 : where i 0 corr and i corr are the corrosion current density without and with the studied inhibitors, respectively. From Table 3, the presence of two cationic aryl bithiophene derivative make the corrosion potential (E corr ) to change to more positive and negative potentials compared with the blank solution. The E corr values were significantly less than 85 mV/SCE, indicating that the studied cationic aryl bithiophene/terthiophene derivatives were mixed-type www.nature.com/scientificreports/ inhibitors [31][32][33] . In addition, the inhibitory activity of cationic aryl bithiophene/terthiophene improved by the presence of lone pairs of electrons and thiophene moiety in these compounds, as well as number of cationic amidine groups. Interestingly, addition of second cationic amidine group improvement outweighs the addition of third thiophene group and this may be attributed to solubility effect of the cationic group. For that reason, cationic aryl bithiophene/terthiophene derivatives can be ordered according to their efficiency as anti-corrosion of API 5L X70-type C-steel in HCl solution as follows: MA-1740 > MA-1615.
Electrochemical impedance spectroscopy (EIS) test. EIS is a widely utilized tool in corrosion inhibition studies of metals 34 . The corrosion behaviour of API 5L X70-type C-steel in HCl solution without and with different doses of the cationic aryl bithiophene/terthiophene derivatives was demonstrated by impedance tests.      Table 5. Nyquist plot in Fig. 7 displayed single capacitive loops, presenting that the dissolution process of API 5L X70 grade C-steel electrode in 1 M HCl solution in the absence and presence of inhibitors undergoes through charge-transfer and double-layer capacitance at the interface of C-steel/solution. As shown in Figs. 7 and 8, the Nyquist and Bode plots show the same style, inferring that the mechanism of cathodic and anodic corrosion reactions was not altered upon addition of cationic aryl bithiophene/terthiophene inhibitors. Whereas, the semicircles diameter becomes a little wider with raising the doses of studied cationic aryl bithiophene/terthiophene derivatives in 1 M HCl corrosive solution 35,36 . In addition, the Bode curves for the studied inhibitors in Fig. 8 exposed that the phase angle and the absolute impedance curves turned wider and larger with the rise in the inhibitor doses; such modifications support the inhibiting action of these inhibitor molecules against the corrosion of API 5L X70 type C-steel in 1 M HCl solution 37 . The equivalent circuit for data fitting experiments was presented in Fig. 9, and the fitted EIS data were given in Table 5. The electrical equivalent circuit includes solution resistance (R s ), charge transfer resistance (R ct ), and the constant phase element (CPE). C dl is the double-layer capacitor. The CPE impedance (Z CPE ) is calculated by the following equations [38][39][40] :  where R ct and R 0 ct denotes the charge transfer resistances for C-steel in inhibited and uninhibited acid test solutions, respectively. The data in Table 5 revealed that R ct values is higher when the dose of cationic bithiophene/ terthiophene additives increases in test solutions. While we notice drop in the values of C dl with uplifting the concentration of inhibitors in acid medium. This action can be correlated to the adsorption of studied cationic bithiophene/terthiophene derivatives on C-steel/electrolyte interface that blocks corrosion reactions on C-steel surface and protect the metal from further acid attack 42 . The data in Table 5 revealed that the lower value of n in the blank solution (free from inhibitor molecules) can be attributed to the attack of aggressive ions on carbon steel surface leading to increase the heterogeneity of the surface. However, in the presence of inhibitor concentrations the values of n increase due to the formation of protection film and increasing the homogeneity of the surface 41,43 .
In view of the results presented in Table 5, the adsorption of the cationic bithiophene/terthiophene derivatives molecules onto the surface of API 5L X70 type C-steel occurs by substituting the existing water molecules and then creating a thicker and more ordered fencing protection layer. This has resulted in their ability to inhibit electron transfer and generally mitigate the corrosion reaction 44,45 . The EIS results revealed that the value of R ct increases gradually with the increase in concentration of the synthesized inhibitors and this indicates an increase in the corrosion inhibition efficiency, which agrees with the potentiodynamic polarization results obtained.
Regression coefficients (R 2 ) is listed in Table 5. R 2 is a statistic that will give some information about the goodness of fit of a model. In regression, the R 2 coefficient of determination is a statistical measure of how well the regression predictions approximate the real data points. An R 2 of 1 indicates that the regression predictions perfectly fit the data. The data in Table 5 revealed that R 2 is an approximation of 1 indicating that the regression predictions fit well with the EIS data.  www.nature.com/scientificreports/ Adsorption study. The inhibition potential of corrosion inhibitor depends on the adsorption behaviour on C-steel surface. To investigate the type adsorption reaction of the cationic aryl bithiophene/terthiophene derivatives on API 5L X70-type C-steel surface, the adsorption isotherms must be investigated. Langmuir adsorption isotherm showed the best fit for the tested cationic aryl bithiophene/terthiophene derivatives that is defined as follows 46 : where K ads is the adsorption equilibrium constant, C represents the inhibitor concentration, and θ is the surface coverage. Figure 10 represents the plotted curves of C/θ versus C for the investigated compounds, which gave a straight line, and the extracted parameters were recorded in Table 6. The linear correlation coefficient (R 2 ) and slope of these linear curves were found close to unity, indicating that the adsorption of these cationic aryl bithiophene/terthiophene derivatives on the surface of API 5L X70 type C-steel in 1 M HCl solution follow the Langmuir isotherm. The Gibbs standard free energy (�G o ads ) , which is a useful thermodynamic parameter in our study was calculated as follows 18,47,48 : where R is the gas constant, T represents the absolute temperature, and K ads is the adsorption equilibrium constant that can be calculated from the interception of the isotherm line with the C/θ axis. The values of K ads and G o ads were listed in Table 6. The higher values of K ads indicate the strong adsorption ability of these studied cationic aryl bithiophene/terthiophene derivatives on the surface of API 5L X70 type C-steel in 1 M HCl solution, and then enhanced protection against corrosion. From the results in Table 6, the values of G o ads are negative, which indicates that the adsorption of the studied inhibitors on C-steel surface is a spontaneous process. Inspecting the obtained values of G o ads in Table 6, the ΔG°a ds values of the studied inhibitors ranged from − 33.88 to − 36.13 kJ mol −1 , so it is the adsorption process occurs through both physical and chemical adsorption type of interactions 49-51 . UV-visible spectrometric measurements. According to the literature reports, the influence of corrosion inhibition on metals in the test media in the presence of an inhibitor may be attributed to the complex formation between a metal and an inhibitor [52][53][54] . In our present work, in order to confirm the possibility of a complex formation between the C-steel electrode and the inhibitor molecules, UV-Vis spectrometric measurements were recorded for (500 µM) of the inhibitor, (500 µM) HCl solution containing (500 µM) of inhibitor after C-steel immersion for 28 h at 30 °C (Fig. 11a,b). The electronic spectra of investigated inhibitors (Fig. 11a,b) show visible peaks at (394, and 335 nm), and (396, and 337 nm) for MA-1615, and MA-1740, respectively. This may be attributed to the π-π* transition with an important charge transfer character. On the other hand, after  Atomic force microscopic (AFM) analysis. Atomic force microscope (AFM) analysis is a powerful surface analysis technique to assess the morphological surface changes for the used substrate prior to and after the incorporation of corrosion inhibitors in test solutions. Figure 12a-c describes the AFM 3D images of polished C-steel surface and the surface of C-steel after 24 h immersion in blank 1 M HCl and another test solution with 0.01 M of inhibitor MA-1740 at room temperature. As shown in Fig. 12a, the API 5L X70 type C-steel before immersion was very smooth and homogeneous surface, while the surface of C-steel after immersion in 1 M HCl without inhibitor displayed a severely damaged and roughed surface as shown in Fig. 12b. Figure 12c  Corrosion inhibition mechanism. The cationic aryl bithiophene/terthiophene derivatives showed outstanding corrosion inhibitive properties which is attributed to the adsorption of these cationic aryl bithiophene/ www.nature.com/scientificreports/ terthiophene derivatives on API 5L X70 type C-steel surface in 1 M HCl through either physisorption or chemisorption type of interaction as presented in Fig. 13. Thiophene rings and nitrogen atoms are considered the active binding sites in the adsorption mechanism. Physisorption involves first the columbic interaction among chloride ions, and then electrostatic interaction takes place between the positively charged N atoms in inhibitors and the negative charge on the chloride ions that adsorbed to the positively-charger API X70 type C-steel surface in acid solution. This interaction forms a thin barrier layer, inhibiting API X70 C-steel surface from reacting with corrosive species (physical adsorption). Moreover, the existence of chloride anions increases the capacity of adsorption compounds on API X70 C-steel surface 3 . The adsorption of these investigated compounds, on the other hand, can occur chemically onto the steel surface through the high electron density cloud of the π-electrons aromatic cycles that possess S heteroatoms, and the imine group (-C=N-) that facilitate the extension of the double bond conjugation on the whole structure enhancing the electron distribution and a more pla- www.nature.com/scientificreports/ nar conformation on the substrate surface 60 . The chemical adsorption is also feasible via the coordinating bonds that can be formed between the lone-paint of electron on the heteroatoms (S) of thiophene rings and the vacant d-orbitals of Fe atoms in API 5L X70 type C-steel surface 61,62 . The variation between the two inhibitor molecules MA-1740, and MA-1615 in terms of their chemical structure feature was responsible for their different corrosion inhibition performance, as reported throughout all this study. Examining these structural deviations, the MA-1740 molecule possessed two amidine groups and two thiophene rings, and MA-1615 molecule possessed three thiophene rings and one amidine group. Accordingly, these cationic aryl bithiophene/terthiophene derivatives can build up a protective film on the surface of API 5L X70 type C-steel via physical and chemical reactions, protecting the X70 C-steel from further dissolution.

Conclusion
1. Two novel cationic aryl bithiophene/terthiophene derivatives MA-1615, MA-1740 were synthesized, characterized by spectral means. 2. The new aryl bithiophene/terthiophene derivatives were investigated as corrosion inhibitors in HCl for API 5L X70 C-steel using electrochemical techniques, which showed higher corrosion inhibition action than similar compounds in structure in literature 3. The adsorption of the two investigated cationic inhibitors on API 5L X70 C-steel surface followed Langmuir isotherm and blocking the corrosion reaction sites. The negative sign of G • ads is an indication that the adsorption of cationic aryl bithiophene/terthiophene derivatives is a spontaneous process. G • ads values means that the adsorption involves both chemisorption and physisorption mechanisms. 4. Tafel curves indicated that the two investigated cationic inhibitors impede the cathodic and anodic reactions simultaneously, and therefore considered as mixed-type inhibitors. EIS measurements indicate that the increase in the inhibitor′s concentration is accompanied with a rise in the values of both R ct and IE % and a decline in the value of C dl . 5. AFM pictures confirmed the presence of protective layer on API 5L X70 C-steel surface via the adsorption of the two investigated cationic bithiophene/terthiophene inhibitors. This protective layer protects API 5L X70 steel surface from further acid attack. 6. The presence of dicationic group in MA-1740 has more structural effect on increasing the corrosion inhibition than introducing third thiophene ring.